This invention relates to quinacridone pigments and a process of producing the same.
Quinacridone pigments are well known organic pigments which are particularly useful as colourants for high molecular weight organic materials.
Of the quinacridone pigments the di-substituted pigments, more particularly 2,9-dichloroquinacridone, are most notable for their pigmentary properties. 2,9-dichloroquinacridone is known to exist in three different polymorphic forms and it is known that each of the particular polymorphic forms have different pigmentary properties. In fact, the polymorphic form displays the best pigmentary properties.
It has now been found that a new polymorphic form of 2,9-dichloroquinacridone can be formed by a synthesis utilizing concentrated sulphuric acid.
The invention provides in one of its aspects a polymorphic form of 2,9-dichloroquinacridone having an x-ray diffraction pattern which comprises a major reflection corresponding to an interplanar spacing (d-value) of 3.20 Angstroms and an associated double glancing angle (Grade 2xcex8) of 27.9.
The new polymorphic form of 2,9-dichloroquinacridone is characterized by a number of reflections in its x-ray diffraction pattern which are not all found in the other known forms of this pigment. They include those reflections having the following interplanar spacings (d-value) and double glancing angles (Grade 2xcex8):
It will be understood that the figures recited above for the d-value and the double glancing angle are of course subject to fluctuation due to experimental error of xc2x10.1 (double glancing angle)
The invention provides in another of its aspects a process of making a 2,9-dichloroquinacridone comprising the step of reacting 2,5-di(4-chloroanilino)terephthalic acid in concentrated sulphuric acid.
The term xe2x80x9cconcentrated sulphuric acidxe2x80x9d includes the acid at a strength of 90 to 95%, especially 92%.
The reaction is carried out at elevated temperatures, between 100 to 130xc2x0 C., more preferably at 110xc2x0 C.
The 2,5-di(4-chloroanilino)terephthalic acid starting material can be conveniently formed from dimethyl succinylosuccinate and 4-chloroaniline according to conventional syntheses employing commonly available reagents.
Preferably the 2,9-dichloroquinacridone formed according to the afore-mentioned reaction is filtered and washed salt-free with water before optionally being dried under vacuum at elevated temperature and optionally used as wet cake.
The 2,9-dichloroquinacridone so formed is in a crude form and is generally not possessed of the requisite desirable pigmentary properties. Accordingly, the crude quinacridone has to be processed further to obtain its desirable pigmentary properties. In a process according to the invention, the crude 2,9-dichloroquinacridone once formed is further processed by milling. Milling is preferably carried out using a ball-mill according to a procedure known in the art. The grinding action is carried out using, e.g. glass balls having a diameter of from 0.6 mm to 0.9 mm.
The 2,9-dichloroquinacridone pigment according to the invention can be applied to polymeric materials and displays better purity of nuance than heretofore achievable for the quinacridone pigments.
The term xe2x80x9cpolymeric materialsxe2x80x9d includes solvent-containing and solvent-free plastics materials, e.g. polyolefin, PVC, polystyrene and acrylic, polyester, alkyd or polyurethane lacquers.
There now follows a series of examples which serve to illustrate the invention.
Synthesis of 2,5-di(4-chloroanilino)terephthalic Acid
A four-necked flask equipped with thermometer, stirrer and condenser is flushed with nitrogen and then charged with 228.2 parts of dimethyl succinylosuccinate, 267.8 parts of 4-chloroaniline and 3.3 parts of 65% sulphuric acid in 890 parts of n-butanol. The reaction mixture is stirred and heated to 110 to 115xc2x0 C. for 3 hours. During this time, 222.5 parts of n-butanol are allowed to distil over and are condensed for later use. After cooling the reaction mixture to 105xc2x0 C., the n-butanol is returned to the reaction mixture followed by 5.3 parts of triethylamine and the condenser is replaced with a reflux condenser. The mixture is cooled to 90xc2x0 C., 150.3 parts of powdered sodium m-nitrobenzene sulphonate is added over a 10 minute period and the mixture heated to reflux. A dropping funnel is introduced to the apparatus and 392.0 parts of 30% sodium hydroxide is added dropwise over a one hour period into the reaction mixture. Refluxing is continued for 2 hours. Thereafter the reflux condenser is replaced with a condenser and 1760 parts of water are added dropwise to the reaction mixture to allow for azeotropic distillation of the n-butanol. The content of the flask is cooled to 70xc2x0 C. and the pH is adjusted to 5 with 304 parts of 35% hydrochloric acid. The resulting slurry is filtered, washed with hot water until the filtrate has a pH of 5 and dried conventionally under vacuum to yield 389 parts of the title compound.
Synthesis of 2,9-dichloroquinacridone
To 937 parts of 92% sulphuric acid at 110xc2x0 C. are added over a period of 3 hours 300 parts of 2,5-di(4-chloroanilino)terephthalic acid. The resultant mixture is heated for 12 hours at 130xc2x0 C. The acid strength is reduced to 75% with the dropwise addition of 203 parts 33% sulphuric acid. The temperature of the resulting suspension is allowed to rise over the next 3 hours to 30xc2x0 C. The solids in the reaction mixture are collected by filtration and washed with 1000 parts of sulphuric acid (75%), 2000 parts of water, 83.2 parts of ammonia (25%) and 2000 parts of water to obtain 385 parts of wet cake which is optionally oven dried under vacuum to yield 224.9 parts of crude 2,9-dichloroquinacridone.
Milling of the Crude 2,9-dichloroquinacridone
A 0.5 liter jar is charged with 17.1 parts of crude 2,9-dichloroquinacridone wet-cake (58.4%=10 g 100%), 30 parts of sodium chloride, 150 parts of acetone and 500 parts of glass beads (diameter 0.6 to 0.9 mm). The jar is sealed and rotated on a roller mill for 72 hours. The mixture is then sieved to separate the balls, filtered and washed with 2000 parts of water. The resultant pigment is dried under vacuum at 100xc2x0 C.
The resultant pigmentary form is characterized by its x-ray diffraction pattern which is shown in FIG. 1 and FIG. 2. The x-ray diffraction pattern can be compared with the x-ray diffraction pattern of the pigmentary form obtained by cyclisation in polyphosphoric acid according to a process described in GB 868 360 and subjected to the milling step described above, see FIG. 3 and FIG. 4.
Cyclisation of 2,5-di(4-chloroanilino)terephthalic acid in polyphosphoric acid as described in the following documents GB 868 360, US 3 257 405, JP 63 199 769 and US 5 496 405 represent the state of the art methods of obtaining 2,9-dichloroquinacridone.